When data from a measurement apparatus, such as a chromatograph or spectrometer, is continuously plotted against time, wavelength, etc., a measurement curve including a peak or peaks is obtained. Due to various factors, the base line of a measurement curve often deviates from the zero line of the measurement apparatus or the recorder. In this case, it is necessary to determine the base line first to measure the true height (or to measure the area) of the peak or peaks. Determining the base line is especially difficult when, as shown in FIG. 1, the base line B of a measurement curve C (on which the peak or peaks P1, P2 stand) curves.
A conventional method of determining a base line is as follows. First a peak is detected by detecting the rising point and the falling point at which the absolute value of the slope of the measurement curve becomes greater than a certain reference value and becomes less than another reference value. The starting point of the peak is determined as the point which is a preset distance before the rising point, and the ending point of the peak is determined as the point which is a preset distance after the falling point. The base line in this case is determined as the line connecting the starting point and the ending point.
There are several other methods of determining the base line of a peak, but, in any case, the starting point and the ending point of a peak should be determined first. In order to determine these points, the operator must determine various parameters beforehand such as the reference values for the slope of the rising point and of the falling point or the values of the preset distances. If the parameters are once determined, then the base line can be determined automatically. But many operators find difficulties in determining appropriate values for the various parameters because the values are not apparent from the measurement curve.